This proposal addresses questions about the development of modulation by neurotransmitters of the action potential of dorsal root ganglion cells of Xenopus tadpoles. My goals are (a) to determine the extent of modulation by GABA, serotonin or enkephalin of the action potential of dorsal root ganglion cells, (b) to determine the timing of the acquisition of neuromodulation by examining the correleation of neuromodulation with (i) the type of action potential expressed by the neuron, (ii) the age of the cell examined, (iii) the developmental stage of the tadpole, (c) to identify the membrane current that is sensitive to modulation, and (d) to investigate the mechanism of this modulation with respect both to developmental properties and roles of likely molecular intracellular components (GTP binding proteins, protein phosphorylation). Blockers and stimulators of these regulatory mechanisms will be used. The studies will be carried out on in vivo preparations of Xenopus tadpoles over a range of ages. Intracellular recording under current clamp conditions will be used to measure membrane potentials, action potentials, input resistance, and membrane potential and conductance changes in response to application of neurotransmitter. The effects of modulators on individual membrane currents (e.g., Na+, Ca2+, or K+ current) from neurons whose intracellular contents from cell- attached patches will be recorded using patch clamp techniques. Cell birthdating will be accomplished by 3H-thymidine labeling and autoradiographic analysis of dorsal root ganglion cells in Xenopus tadpoles whose exposures to label are begun at different stages between 1.5 and 45 days of embryonic development.